isochasm		a locus of points where auroras are sighted with equal frequency.
almucantar		(astron.) the circle of all points at a specified altitude on the celestial sphere.
aclinic line		the locus on the earth's surface, approximating the equator, where a compass needle does not tend from the horizontal.
agonic line		the locus of all points on the earth's surface whose magnetic declination is zero. [< Gk /go~nia~/ angle]
thalweg		a imaginary line along the bottom of a valley or riverbed.
hachure		the same as a contour line?